1. Field of the Invention
This invention relates generally to signal transmission methods, and more particularly to acoustic data telemetry methods for transmitting data from a downhole location to the surface.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached at a drill string end. Modern directional drilling systems generally employ a drill string having a bottomhole assembly (BHA) and a drill bit at end thereof that is rotated by a drill motor (mud motor) and/or the drill string. A number of downhole devices in the BHA measure certain downhole operating parameters associated with the drill string and the wellbore. Such devices typically include sensors for measuring downhole temperature, pressure, tool azimuth, tool inclination, drill bit rotation, weight on bit, drilling torque, etc. Downhole instruments, known as measurement-while-drilling (“MWD”) and logging-while-drilling (“LWD”) devices in the BHA provide measurements to determine the formation properties and formation fluid conditions during the drilling operations. The MWD or LWD devices usually include resistivity, acoustic and nuclear devices for providing information about the formation surrounding the borehole.
Downhole measurement tools currently used often, together and separately, take numerous measurements and thus generate large amounts large amounts of corresponding data. Due to the copious amounts of these downhole measurements, the data is typically processed downhole to a great extent. Some of the processed data must be telemetered to the surface for the operator and/or a surface control unit or processor device to control the drilling operations. For example, this processed data may be used to alter drilling direction and/or drilling parameters such as weight on bit, drilling fluid pump rate, and drill bit rotational speed. Mud-pulse telemetry is most commonly used for transmitting downhole data to the surface during drilling of the borehole. However, such systems are capable of transmitting only a few bits of information per second, e.g., 1-4 BPS. Due to such a low transmission rate, the trend in the industry has been to attempt to process greater amounts of data downhole and transmit only selected computed results or “answers” uphole for controlling the drilling operations. Still, the data transmission requirements far exceed the capabilities the current mud-pulse and other telemetry systems.
Acoustic telemetry systems have been proposed for higher data transmission rates. Piezoelectric materials such as ceramics began the trend, and advancements in the use of magnetostrictive material has potentially enabled even more efficient transmitting devices. These devices operate on the general concept of creating acoustic energy with an actuator having one of the above materials.
The created acoustic energy is modulated in frequency, phase, amplitude or in any combination of these, so that the acoustic energy contains information about a measured or calculated downhole parameter of interest. The acoustic energy is transferred into a drillstring thereby setting up an acoustic wave signal. The acoustic signal propagates along the drillstring and is received by a receiver. The receiver is coupled to a controller for processing and/or recording the signal. In deep well applications, there may be one or more intermediate transmitters disposed along the drillstring to facilitate signal transmission over the longer distance.
Although acoustic telemetry provides data rate benefits not capable in mud-pulse telemetry, conventional acoustic telemetry methods suffer from physical limitations existing within the transmission medium, i.e., the drillstring. In particular, a drill pipe having jointed pipes pose special problems for the conventional methods of acoustic transmission.
Due to necessarily repetitive spacing of tool joints within the drillstring, the drillstring exhibits certain acoustic properties. One of the most important of these is the presence of frequency bands in which there is severe attenuation of acoustic signals. These frequency bands occur repetitively in the frequency spectrum (rather like the tines on a comb) and are referred to as stopbands. The intervals in between these stopbands are referred to as passbands. Acoustic energy may be transmitted along the drillstring when the signal frequency is within one of the passbands.
A known method of transmitting a message signal along the drillstring is using pulses of acoustic energy to represent the digital information. This is a form of telemetry using amplitude modulation (also referred to as ASK or Amplitude Shift Keying) to encode information about the downhole parameter of interest. Exemplary methods include the use of signal switching between “off” and “on” states to represent binary states, or the use of high amplitude, broad frequency bandwidth, “shock” pulses. These methods suffer from high error or data “drops” and low transmission rates caused by the inability of receiving and processing circuits to distinguish the data signals. This is due to high levels of background noise caused by drilling vibrations, or to echoes of the transmitting signals within the drillstring.
The present invention addresses the drawbacks identified above by determining one or more frequency ranges for natural stopbands of a drill string and selecting a modulating frequency based on the frequency ranges of the stopbands for transmitting data signals.